Required properties of automobile tires include fuel efficiency, durability, gription, and the like. As polymer compositions used for automobile tires to increase these properties, many conjugated diene polymer compositions have been proposed that contain a conjugated diene polymer such as polybutadiene or a styrene-butadiene copolymer, and a reinforcing agent.
For example, Patent Literature 1 proposes a polymer composition in which silica as a reinforcing agent is added to a polymer obtained by homopolymerizing butadiene or copolymerizing butadiene and styrene in a hydrocarbon solvent using n-butyllithium as a polymerization initiator. Patent Literature 2 proposes a polymer composition in which silica and carbon black as reinforcing agents are added to a polymer obtained by copolymerizing butadiene and styrene in a hydrocarbon solvent using n-butyllithium as a polymerization initiator.